I just noticed today that Transport for London is testing fuel cell buses on a certain route (RV1 - Covent Garden to Tower Gateway). The buses visibly advertise "zero emissions" -- right above a tailpipe which is clearly emitting something...

This emission is water vapour, as stated on the TFL website:

The only emission from the fuel cell bus is water, which forms a vapour cloud as it leaves the exhaust and enters the atmosphere.

And we all know, water vapour is one of the most potent greenhouse gases. Perhaps someone should contact the UK's Advertising Standards Agency with regard to the false claims made on these buses...